No abstract

Monoanionic ligands of the type [R 2 P(E)NP(E)R 2 ] À 1 have been investigated extensively as ligands for both main group elements [1] and transition metals. [2] This widespread interest stems from their potential uses as lanthanide shift reagents, [2] industrial catalysts, [3] luminescent materials, [4] or in metal extraction processes. [5] Recently we [6] and Stahl et al. [7] reported the first ambidentate dianionic ligands [RN(E)P(m-NR) 2 P(E)NR] 2À 2, which adopt a variety of bonding modes, that is N,E, N,N', or E,E', with metal centers.Despite this intense activity, the tellurium analogues of 1 and 2 are unknown. Anionic tellurophosphinic amides of the type [tBu 2 P(Te)NR] À (R ¼ iPr, Cy) can be prepared by lithiation of [tBu 2 P(Te)NHR] with LinBu, and chelate complexes of this anion with Group 12 metals have been investigated as single-source precursors of binary metal tellurides. [8] Although ditellurides of the type [R(Te)P(m-NtBu) 2 P(Te)R] (R ¼ Me, tBu) have been reported, [9] our attempts to oxidize tBuN(H)P(m-NtBu) 2 PN(H)tBu with an excess of elemental tellurium in boiling toluene produced only the monotelluride [tBuNH(Te)P(m-NtBu) 2 PN(H)tBu] in about 5 % yield. [10] Endeavors to generate (TePPh 2 ) 2 NH in a similar manner have also been unsuccessful. Consequently, we adopted a different approach to the synthesis of the anionic ligands 1 (E ¼ Te) and 2 (E ¼ Te), which involves metalation of the neutral imido or amido precursor prior to the reaction with tellurium. [11] Herein, we report the synthesis and X-ray structures of [{[Na(tmeda)][(TePPh 2 ) 2 N]} 2 ] (3) and [Li(tmeda)] 2 [Te(NtBu)P(m-NtBu) 2 P(NtBu)Te] (4), (tmeda ¼ tetramethylethylenediamine), which contain the first examples of 1 (E ¼ Te) and 2 (E ¼ Te), respectively.

The effect of subtle changes in the sigma-electron donor ability of 4-substituted pyridine ligands on the lead(II) coordination environment of (2,6-Me(2)C(6)H(3)S)(2)Pb (1) adducts has been examined. The reaction of 1 with a series of 4-substituted pyridines in toluene or dichloromethane results in the formation of 1:1 complexes [(2,6-Me(2)C(6)H(3)S)(2)Pb(pyCOH)](2) (3), [(2,6-Me(2)C(6)H(3)S)(2)Pb(pyOMe)](2) (4), and (2,6-Me(2)C(6)H(3)S)(2)Pb(pyNMe(2)) (5) (pyCOH = 4-pyridinecarboxaldehyde; pyOMe = 4-methoxypyridine; pyNMe2 = 4-dimethylaminopyridine), all of which have been structurally characterized by X-ray crystallography. The structures of 3 and 4 are dimeric and have psi-trigonal bipyramidal S(3)N bonding environments, with the 4-substituted pyridine nitrogen and bridging sulfur atoms in axial positions and two thiolate sulfur atoms in equatorial sites. Conversely, compound 5 is monomeric and exhibits a psi-trigonal pyramidal S(2)N bonding environment at lead(II). The observed structures may be rationalized in terms of a simple valence bond model and the sigma-electron donor ability of the 4-pyridine ligands as derived from the analysis of proton affinity values. Solid-state (207)Pb NMR experiments are applied in combination with density functional theory (DFT) calculations to provide further insight into the nature of bonding in 4, 5, and (2,6-Me(2)C(6)H(3)S)(2)Pb(py)(2) (2). The lead chemical shielding (CS) tensor parameters of 2, 4, and 5 reveal some of the largest chemical shielding anisotropies (CSA) observed in lead coordination complexes to date. DFT calculations using the Amsterdam Density Functional (ADF) program, which take into account relativistic effects using the zeroth-order regular approximation (ZORA), yield lead CS tensor components and orientations. Paramagnetic contributions to the lead CS tensor from individual pairs of occupied and virtual molecular orbitals (MOs) are examined to gain insight into the origin of the large CSA. The CS tensor is primarily influenced by mixing of the occupied MOs localized on the sulfur and lead atoms with virtual MOs largely comprised of lead 6p orbitals.